Why are there no green mammals? – Mail


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The color green is usually associated with the concept of nature: green are the leaves of plants, as well as many animals, including various insects, molluscs and vertebrates, especially fish, amphibians, reptiles and birds. However, mammals, a class of vertebrates that includes humans, are not green: a peculiarity that over time has piqued the curiosity of readers of popular science magazines and visitors to Internet forums.

To the question “why are there no green mammals?” cannot be answered clearly. The question can be considered from different points of view: from the point of view of considerations of the evolutionary advantages of having hair or skin of one color over another, and from the point of view of the physical properties of hair, which is one of the attributes that distinguishes mammals from other animals.

The colors of the outermost parts of animal bodies, whether bare skin, scales, feathers or hair, can be caused by two different physical mechanisms. “One of them is the presence of pigments inside the cells,” explains Adriano Martinoli, zoologist specializing in mammals and professor at the University of Insubria, as well as co-author of the podcast Post to foreign species Near and far. Pigments are colored substances that determine the color of a fabric. “And the color of pigments present in some cells can mix, not physically, but visually, with the color of pigments present in other cells to produce new colors”.

Chlorophyll is a green pigment, a substance present in leaf cells that absorbs some of the sun’s energy that powers plants. In autumn, when the hours of light decrease, the chlorophyll-containing cells of many plants become less vital and gradually reduce the photosynthesis of chlorophyll: as a result, the leaves change color because other substances, previously covered by the green of chlorophyll, are visible. . For example, carotenoids have warm colors that vary from red to yellow. The pigments contained in human skin (as well as in hair) are melanins: the color changes depending on the amount and type of these substances, factors that are influenced by genes and exposure to sunlight.

However, the animal’s color can also be caused by something more complex, i.e. “a surface physical microstructure that reflects light in a certain way”, Martinoli explains: “For example, the coloration of the scrotum of many primates during the reproductive period is not caused by pigment, but by the reflection of light. In fact, there are microstructures in the cells of the epidermis that, by reflecting light, make the surface of the skin appear to have a certain color, which is not the case”.

This is the case with the skin of mandrills, primates from Central West Africa known for the bright colors of their faces, red and blue. Red is due to hemoglobin, a red-colored protein present in blood (and therefore a pigment), while blue has a different origin. Blue pigments are very rare in nature, and in the case of mandrills, the blue is produced by the arrangement of collagen fibers in their skin (collagen is itself a protein). It’s a mechanism that also affects the colors of the feathers of many colorful birds: they don’t contain color pigments, so when you look at them magnified under a microscope, you see them white and brownish.

A female mandrill and her cub in the zoo

A female mandrill and her cub at the New Orleans Zoo, United States, in 2020 (AP Photo/Gerald Herbert)

The physical phenomenon responsible for these colors is similar to why the daytime sky appears blue.

Sunlight is electromagnetic radiation and is composed of waves of different frequencies. Each corresponds to a different color in the spectrum, which goes from red to violet and through orange, yellow, green and blue. When light passes through the atmosphere, not everything is scattered in the same way: light corresponding to higher frequencies is scattered much more due to how the particles of the atmosphere are made, which is why we see a blue sky because the light that reflects off them and what comes to us is especially this color. Violet also corresponds to a high frequency, but the sun emits more blue light than violet.

Something similar happens with the feathers of birds or with the skin of some animals: in the latter case, the microscopic structure of collagen is related.

The physical structure of hairs, which are essentially uncomplicated tubes of keratin, does not allow this type of effect to occur, unlike the more complex feathers of birds. And in terms of pigments, it can only contain different types of melanin, which give colors that vary from yellow to dark brown. For example, pheomelanin gives shades between yellow and reddish, while eumelanin is responsible for dark brown, which in some cases approaches black. When the hairs are white, it means that they do not contain pigments, and gray is a mixture of black and white. However, the red fur of some mammals differs from the lighter plumage color of some birds, which is instead caused by a type of pigment that mammals do not have: carotenoids.

– Read also: Why does hair turn white?

To this reflection, more closely related to physics, we can add considerations of the evolutionary advantages associated with color based on what we know about the history of mammals. Mammals come from the reptile family, as do birds. As Martinoli explains, this group probably already lacked the ability to produce some pigments, so it was not inherited. Or it could be that the ability was there but was lost in the course of evolution because it wasn’t useful, that is, it didn’t provide adaptive advantages.

In fact, mammals originally occupied “biological niches”: in a world in which reptiles were the most widespread dominant animals on Earth, mammals lived in the few contexts that remained free. They were mostly small or very small animals active at night. For this reason, it is likely that they did not need to have a flamboyant appearance and bright colors, as well as refined eyesight: smell and hearing were more important senses.

There is also another aspect, which is that most mammals have black and white, dichroic vision. The presence of colors would not be useful for mating, as it happens in many species of birds, where males are also attracted to females due to the quality of their plumage.

The colors of mandrills, which are primates (and therefore mammals), are explained by the peculiarities of the primates themselves. “Because of the random occurrence of eye mutations, primates are an exception and have trichromatic vision, that is, they see colors,” continues Martinoli. “And it seems that in ancestral primates, this color vision was the key to success because it allowed them to distinguish ripe fruits very well.” That would be a big evolutionary advantage.

– Read also: Nightlife saved mammals from dinosaurs

As for camouflage, i.e. the ability to blend in with the environment, colors close to brown are suitable for many small mammals in order not to stand out on the ground and in the vegetation. And even furs that may seem garish to us, such as tigers, are actually good for camouflage if you want to hide from animals (in this case, prey) that have dichromatic vision.

However, in a sense, there are mammals with green fur, even if it is not fur correctly green. They are sloths, animals known for their slow movements that live in trees in some areas of Central and South America: their hair grows algae that carry out photosynthesis, so they are green and give this shade to the coat of sloths. Scientists believe that the presence of algae is advantageous: firstly because thanks to it it is better camouflaged between the leaves of trees and hides from predators, secondly because it is an additional source of food.

– Read also: Why are sloths so slow?


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